Electromagnetically actuated spring clutches are well known in the art of paper feeding devices, as used in electrophotographic printers, copiers, facsimile machines, and the like. Typically, such a machine will include a main drive which rotates continuously when the machine is "on," whereas rotational motion for the output shaft is required only intermittently, as, for example, in moving individual sheets through a paper feed apparatus. To obtain this intermittent rotational motion from the continuous rotational motion of the main drive, an electromagnetic clutch is employed for selective engagement of the output shaft by the input hub. Within the clutch, a helical, torque-transmitting clutch spring carried by the input hub rotates about the output shaft. When a magnetic field is applied to the clutch spring, as by an external magnetic coil, the helical spring is caused to wrap down on and engage the sides of the output shaft, so that the rotational motion of the input hub is transmitted through the clutch spring to the output shaft. The clutch can be engaged and disengaged relatively rapidly by means of selectively energizing the magnetic coil.
A typical prior art magnetic clutch of a design commonly used in the art of paper-feeding devices is shown in radial cross-section in FIG. 1.
The input hub 10 rotates coaxially with the output shaft 12. Attached to input hub 10 is a fixed end of helical spring 14. (As this is a radial cross-sectional view, the individual turns of spring 14 are shown end-on, in cross-section, in FIG. 1.) Input hub 10 carries along helical spring 14 by its fixed end, so that a certain number of turns of the helical spring 14 near its free end 16 are disposed adjacent the side surface of output shaft 12. At the very tip of free end 16 of spring 14 is a tang 18.
Disposed around spring 14 is a source of electromagnetic flux, preferably an electromagnetic coil 20, here shown in cross-section. The coil 20 is at least partially enclosed by a housing 22, at least a portion of which is conductive of electromagnetic flux. One portion of housing 22 extends toward a bearing 24, which allows relative motion between the housing 22 and the output shaft 12. Bearing 24 is typically made of plastic or a nonmagnetic metal such as brass. Adjacent a surface of the housing 22 is a shaft flange 30, which is conductive of magnetic flux and extends around the outer circumference of output shaft 12 at a place adjacent the free end 16 of spring 14.
When electromagnetic coil 20 is energized, electromagnetic flux passes through the clutch in the path indicated by the bold arrows. The flux passes through housing 22 and then through shaft flange 30. The flux in shaft flange 30 causes the free end 16 of spring 14 to pull axially toward shaft flange 30, and this axial pull causes the turns at free end 16 of spring 14 to wrap down and engage the side surface of output shaft 12, thus engaging the clutch and permitting transference of rotational motion from the input hub 10 to output shaft 12.
In order to effect the axial pull of free end 16 of spring 14 toward shaft flange 30, two techniques are generally used. One possibility is to make spring 14 of a material conductive of electromagnetic flux, so that flux in shaft flange 30 will cause attraction between shaft flange 30 and the spring 14 itself. However, in practice it has been found that metal alloys which are effective for durable helical springs tend to have unsatisfactory magnetic properties, and vice-versa. One preferred method is to employ a control collar 32, which is an axially-movable hollow cylinder around spring 14, to act as a conduit for magnetic flux passing through the shaft flange 30. Control collar 32 preferably includes a slot 34 defined therein, to accept the tang 18 at the tip of the free end 16 of spring 14. As shown in FIG. 1, magnetic flux flows through the control collar 32 and passes over an axial air gap to housing 22, completing a circuit around coil 20. Even though control collar 32 moves axially with the spring 14 as the clutch is engaged and disengaged, an air gap is preferably always maintained between control collar 32 and housing 22, so as to avoid frictional contact at the interface when the clutch is in operation.
One significant problem which has been experienced with magnetic clutches of this and similar designs results when points of physical wear are also points along the magnetic flux path. Whenever a magnetic flux passes between two objects, such as between housing 22 and shaft flange 30, or between shaft flange 30 and control collar 32, an attractive magnetic force will exist between the two objects; this attractive force is what pulls the control collar 32 toward shaft flange 30. The magnitude of the attractive force is dependent on the square of the magnetic flux density, times the surface area of contact between the surfaces and a constant associated with the magnetic properties of the materials. If the two objects are in contact and moving laterally relative to each other, such as the rotating shaft flange 30 adjacent the stationary housing 22, a drag force exists equal to the magnetic attractive force times the coefficient of friction between the objects. Minimizing such forces in magnetic wrap spring clutches is necessary to extend clutch life and performance.
In the clutch of FIG. 1, the key wear area is the interface between housing 22 and shaft flange 30, where friction between these surfaces creates wear debris. Alternatively, the clutch may be dimensioned such that the shaft flange 30 bears axially against the bearing 24, to leave an air gap between shaft flange 30 and housing 22. This alternative arrangement reduces the frictional force and may provide for a better combination of wear resistant materials. However, an air gap between shaft flange 30 and housing 22 will not entirely eliminate wear forces and wear debris, and will decrease the magnetic force available for clutch activation. The wear debris will tend to migrate across the shaft flange 30, as shown by the black arrow in FIG. 1, and accumulate in the space between shaft flange 30 and the edge of the control collar 32. Ordinarily, de-energizing of the coil 20 causes the spring 14 to retract from shaft flange 30 and unwrap from output shaft 12 as the flux from the shaft flange 30 decays. However, a contamination of debris around the control collar 32 may interfere with the retraction of spring 14, and the clutch will be permanently stuck in an engaged mode.
In order to obviate the problems associated with wear areas in the flux path, numerous schemes have been proposed in the prior art. Reell Precision Manufacturing Corp., of St. Paul, Minn., manufactures a clutch with a "balanced radial flux path, " as disclosed in U.S. Pat. No. 4,263,995 to Wahlstedt. An example of such a clutch is shown in FIG. 2, where like reference numerals from FIG. 1 indicate like elements. Here, the interface between the stationary housing 22 and the rotatable output shaft 12 includes a flange 40 and a nonmagnetic bearing 42. The flange 40 and bearing 42 are rigidly attached to the shaft 12 and rotate therewith. Although the flange 40 is in direct contact with the housing 22, flange 40 is homologous in function to the shaft flange 30 of the clutch of FIG. 1, in that the free end 16 of spring 14 is attracted toward it when the clutch is energized. A key distinction between this clutch and that of FIG. 1 is that the interface between the stationary housing 22 and the movable flange 40 in this clutch is an axial surface, and the flux path across this interface is radial with respect to the output shaft 12.
The flange 40 can be seen to conduct magnetic flux from the housing 22 to a control ring 44, which is attached to the free end 16 of spring 14. This control ring 44 performs the same function as the control collar 32 in the previous example, but control ring 44 has a much shorter axial length. Adjacent control ring 44 is a nonmagnetic bearing 46, which rotates with the output shaft 12. The gap between flange 40 and control ring 44 is appreciable even when control ring 44 is magnetically attracted toward flange 40; control ring 44 and flange 44 preferably never come in physical contact.
The clutch of FIG. 2 further includes an inner coil housing 48, which is stationary, and serves to complete the flux path around coil 20. There is a significant air gap between control ring 44 and inner coil housing 48, and thus there is no wear between these surfaces. More significantly, the flux path across the gap between control ring 44 and inner coil housing 48 is radial with respect to the output shaft 12. Thus, there are two radial portions of the flux path: the interface between housing 22 and flange 40, and the interface between control ring 44 and inner coil housing 48. The first of these portions of the flux path is radial and inward, and the second is radial and outward: these form the "balanced radial flux path" characteristic of this type of clutch. This design has been shown to have the advantages of avoiding any particular wear spots along the circumference of the output shaft 12, and also minimizing magnetic migration of wear debris within the clutch, thereby increasing the reliability.
Although the clutch of FIG. 2 has proven effective in avoiding certain problems associated with reliability in magnetic clutches, further improvements are possible, and in the art of paper feed devices, desirable.
It is one object of the present invention to provide a magnetic clutch which improves on the wear characteristics of magnetic clutches of the prior art.
It is another object of the present invention to provide such an improved magnetic clutch which diverts at least a portion of the flux path from crucial areas of physical wear.
It is another object of the present invention to provide such an improved magnetic clutch which minimizes wear by facilitating balanced axial magnetic forces within the clutch.
Other objects will appear hereinafter.